Thermal Stability for Exhaust Emissions Treatment Materials

ABSTRACT

A storage vessel for emission control liquids such as aqueous solutions of urea is equipped with phase change materials that are calibrated to help maintain the liquid within a proper temperature range. Multiple phase change materials having phase transition temperatures of fifty to seventy degrees Celsius and seventy to one hundred degrees Celsius provide staged high-temperature stability for the urea solutions. Phase change materials can be included that also provide low temperature stability by changing from liquids to solids when freezing of the aqueous solution is threatened. Containers of phase change materials are applied to the interior or exterior surfaces of the storage vessel. Alternatively the storage vessel is included within and surrounded by a housing that forms an annular space with the storage vessel, and phase change materials are installed within the annular space. A method for using the storage vessel in an emissions control system also is presented.

FIELD OF THE INVENTION

This invention provides improved thermal stability for liquids used to reduce exhaust emissions particularly of nitrogen oxides in exhaust gases from diesel engines and lean-burn engines.

BACKGROUND OF THE INVENTION

Diesel engines produce exhaust emissions of oxides of nitrogen (NOx) that must meet new federal and state standards. Reducing these oxides to acceptable levels requires Selective Catalytic Reduction (SCR), which consists of injecting a strong reducing agent into the exhaust stream ahead of the SCR catalyst. The reducing agent and the catalytic action combine under controlled conditions to reduce NOx emissions to acceptable levels.

Ammonia has come to the forefront as the reducing agent of choice, but transporting and using ammonia creates new health and safety issues of its own, especially when one considers that trucks and many cars would be carrying a supply of ammonia on board. The art turned to making up an aqueous solution of urea that is quite safe, and the solution can be hydrolyzed simply by injecting it into an exhaust stream having a suitable temperature that will convert the urea into ammonia, or by passing the solution over a catalyst just prior to injection into the exhaust stream. This minimizes health and safety risks as only minimal amounts of ammonia are present at any particular time.

Aqueous solutions of thirty-two percent urea by weight, the more common concentration, freeze at minus eleven degrees Celsius, a winter temperature not uncommon in the United States and many other nations and frequently encountered by trucks and other vehicles. Additionally an aqueous urea solution begins to decompose at temperatures above fifty degrees Celsius. Decomposition is hastened by temperatures of ninety to one hundred degrees Celsius, and the nozzle or nozzles used to inject the solution into the exhaust stream can be exposed to temperatures well above this range and can become clogged and plugged by decomposing solution.

The art proposes several solutions to preventing decomposition of the solution within the injection nozzle. In U.S. Pat. No. 5,884,475, Hoffman et al. present a high pressure air system that blows any remaining solution out of the nozzle and its lines when the engine is shut down. In U.S. Pat. Nos. 5,976,475 and 6,063,350, Peter-Hoblyn et al. and Tarabulski et al. respectively present re-circulating systems that pump excess aqueous solution of urea to the dosing unit nozzle and return the excess to the storage vessel, thereby using the re-circulated solution to help cool the temperature of the solution within the nozzle.

Tarabulski et al. also propose an electrical heater in the storage vessel that is activated by a control system when environmental temperatures could result in freezing, but the supply of electrical energy for a truck that is stopped for any length of time is finite and maintaining a considerable volume of the solution above freezing can use too much electrical energy. Accordingly Hoffman et al. propose enabling the main portion of the solution to freeze while maintaining a small amount in a heating mechanism that can be thawed and ready for injection soon after a cold start. Patent publication 2007/0157602A provides a different structure that achieves a similar result.

These systems require bulky hardware new to diesel powered vehicles. Packaging the new hardware offers limited options such as squeezing hardware into the engine compartment where it would be subjected to elevated temperatures, mounting it behind the cab of the truck where it would be exposed to the fluctuating environment, or installing the urea storage vessel within the existing fuel tank of the vehicle. Minimizing the lengths of the lines that deliver treatment liquid to the exhaust system also is desirable, which suggests packaging hardware proximately to the engine exhaust system where the treating liquid could be exposed to elevated temperatures or to a cold environment. Complex control systems also are required to heat or cool the treating liquid as necessary.

SUMMARY OF THE INVENTION

This invention represents a different approach to maintaining the thermal stability of aqueous solutions of urea in emission control systems. The invention provides a great deal of flexibility in regard to the different locations of the required additional hardware and can be used in conjunction with systems of the prior art.

The invention provides a storage vessel that comprises an inlet for admitting a liquid into the storage vessel, an outlet for providing liquid to exhaust emissions equipment of the engine, and phase change material in heat transfer relationship with the liquid, said phase change material being calibrated to exchange its heat of fusion with the liquid and thereby stabilize the temperature of the liquid within an identified temperature range.

The phase change material can be calibrated to undergo a phase change and either absorb or release thermal energy, its heat of fusion, when the temperature of the liquid increases above, or decreases below, selected temperatures. Phase change materials (PCMs) are chemical formulations that transform from one phase, from the solid phase to the liquid phase or the reverse from the liquid phase to the solid phase, upon exposure to the phase transition temperature (PTT) of the formulation. During transformation the phase change material absorbs or releases its fusion energy and in this invention transfers that energy to its surroundings, thereby helping to stabilize temperatures of nearby materials. By selecting and calibrating PCMs that are liquids at normal operating temperatures and have a phase transition temperature at lower temperatures, the PCM will yield up its heat of fusion and can be used to reduce and minimize temperature decreases.

Phase change material preferably is installed within one or more sealed containers positioned on the inner surfaces or the bottom of the storage vessel where the container and the PCM it contains contacts and is in a heat transfer relationship with the liquid within the storage vessel. The container usually is at least partially submerged in the liquid to achieve good heat transfer characteristics with the liquid. An alternative installation suitable for aftermarket use applies the containers to the exterior surfaces of the storage vessel. Adding multiple containers with PCMs having the same or differing PTTs provides additional effectiveness and flexibility.

Aqueous solutions of urea preferably are used as the treating liquid, and to help maintain the temperature of the solution within a desired range one or more containers of PCMs installed in the storage vessel has a phase transition temperature of less than one hundred degrees Celsius, at which temperature severe decomposition of urea solutions can occur. PCMs calibrated with PTTs of seventy to one hundred degrees Celsius can be installed to help with elevated temperatures. PCMs calibrated within a lower temperature range of fifty to seventy degrees Celsius can be used separately or in conjunction with PCMs calibrated with higher PTTs to provide staged thermal stability. Containers can have special surface features, both internally and externally, to enhance heat transfer characteristics with the PCMs and the treating solution.

A special embodiment provides a storage vessel that combines the foregoing structures by comprising multiple phase change materials, one that is calibrated with a phase transition temperature above a selected temperature to absorb thermal energy when the temperature of the liquid rises above the selected temperature, and another phase change material that is calibrated with a phase transition temperature below a selected temperature to release thermal energy when the temperature of the liquid falls below the selected temperature. This embodiment provides increased stability by maintaining the liquid within a prescribed temperature range.

An alternate structure of a storage vessel that provides enhanced thermal stability for the liquid it contains comprises an inner storage vessel for receiving and dispensing the liquid, a larger outer housing surrounding and encasing the inner storage vessel and forming an annular space between the outer housing and the inner vessel, and phase change materials within the annular space and in heat transfer relationship with the liquid. The phase change material can be calibrated with a phase transition temperature at a selected temperature to change phase and thereby absorb thermal energy when the temperature of the liquid increases beyond a selected temperature, or change phase to release thermal energy when the temperature of the liquid decreases below a selected temperature, or both.

Substantially filling the annular space with PCM enables the use of additional PCM, and the annular space offers opportunities for PCMs with differing phase transition temperatures to provide staged thermal stability of the liquid for multiple ranges of high temperature control and also for control of low temperatures.

When using this invention with the re-circulating systems taught by Hoblyn et al. and Tarabulski et al., the lines that return aqueous solution heated in the dosing unit nozzle can be packaged within phase change materials calibrated to absorb thermal energy when the aqueous solution is above a desired temperature range. Similarly the lines that deliver aqueous solution to the dosing unit nozzle can be packaged within phase change materials calibrated to either deliver thermal energy to, or absorb thermal energy from, the aqueous solution as required by the system.

The invention also provides a method for injecting a stabilized exhaust treatment liquid into engine exhaust emissions equipment. The method comprises providing a storage vessel for the exhaust treatment liquid, introducing exhaust treatment liquid into the storage vessel, stabilizing the exhaust treatment liquid with phase change material being calibrated to undergo a phase change and absorb thermal energy when the temperature of the exhaust treatment liquid increases above a specified temperature, when the temperature of the liquid decreases below a specified temperature, or both, and conducting the exhaust treatment liquid to the engine emissions equipment.

Calibrating the phase change material in the method with a phase transition temperature within a range of seventy to one hundred degrees Celsius to undergo a phase change and absorb thermal energy when the temperature of the exhaust treatment liquid within the storage vessel is within the range of seventy to one hundred degrees Celsius provides high-temperature control. Staged control can be achieved in the method by placing second phase change material within the storage vessel and in heat transfer relationship with the exhaust treatment liquid, and calibrating said second phase change material to undergo a phase change and absorb thermal energy when the temperature of the exhaust treatment liquid within the storage vessel is fifty to seventy degrees Celsius.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a storage vessel of this invention that has two containers of phase change material installed on the inside surfaces of the storage vessel.

FIG. 2 is a perspective of a container of phase change material ready for installation in the storage vessel of FIG. 1.

FIG. 3 is a top view of the storage vessel of FIG. 1.

FIG. 4 is a top view of a storage vessel similar to that of FIG. 1 but with containers of phase change material installed on the outer surfaces of the storage vessel.

FIG. 5 is a sectional view of an alternate structure in which an inner storage vessel is surrounded by an outer housing to form one or more annular spaces for phase change material.

FIG. 6 is a sectional view of the storage vessel of FIG. 5 that shows a horizontal partition within the annular space.

FIG. 7 is a top view of the storage vessel of FIG. 5 that shows a vertical partition within the annular space.

FIG. 8 is a schematic of an exhaust emissions system that includes a storage vessel of this invention.

FIG. 9 is a sectional of a return line, also applicable to a delivery line, for an exhaust emissions system of this invention.

DETAILED DESCRIPTION

Referring to FIGS. 1, 2, and 3, a storage vessel 10 comprises a cylindrical body 12 and a sealable cap 14 that fits on top of and seals body 12. An inlet 16 in cap 12 enables adding an aqueous solution of urea 17 to storage vessel 10 and an outlet pipe 18 extends through cap 14 and into body 12, terminating near the bottom of body 12. Outlet pipe 18 includes a liquid pump 19 for pumping the aqueous solution out of the storage vessel. Storage vessel 10 and cap 14 can be made of a variety of materials including stainless steel and polymeric materials such as high density polyethylene.

Heating elements, powered for example by electrical energy from a vehicle battery, and cooling equipment, powered for example by AC systems of the vehicle, along with temperature sensors and other control equipment, can be added to storage vessel 10 as needed. These are taught by the prior art and are not included in the drawings of this invention.

Applying attention to FIG. 2, the Figure shows a container 20 that has a curved shape and conforms to the cylindrical walls of body 12. An opening 22 enables adding phase change material 24 to container 20 and closing opening 24 seals the phase change material 24 within container 20. As shown in FIGS. 1 and 3, containers 20 a and 20 b are attached to the inner walls of body 12 where container 20 a contains phase change material 24 a and container 20 b contains phase change material 24 b. Phase change materials 24 a and 24 b are in heat transfer relationship with the aqueous solution of urea 17.

Typically excessive heating of the solution is of major concern, and phase change materials 24 a and 24 b are calibrated to help maintain the temperature of the solution below its decomposition temperature but within its operating range. Accordingly phase change material 24 a in container 20 a is calibrated with a phase transition temperature within a temperature range of fifty to seventy degrees Celsius. As solution temperature approaches this range, phase change material 24 a transforms from solid to liquid and absorbs its heat of fusion from the aqueous solution, thereby passively and effectively helping to maintain the solution below its deterioration temperature and within the desired range.

Staged control is obtained by calibrating phase change material 24 b with a phase transition temperature within a higher temperature range of seventy to one hundred degrees Celsius to transform from solid to liquid within this higher temperature range. During extended operations, as the temperature of the aqueous solution absorbs additional thermal energy, phase change material 24 b also transforms to help maintain the temperature of the aqueous solution below its deterioration temperature. Additional containers of phase change material can be installed within storage vessel 10 and with phase change materials also calibrated to transform at temperatures within the desired operating range and below severe decomposition temperatures of aqueous solution 17.

In other installations where cooling harmful to use of the aqueous solution can occur, containers 24 can contain phase change material 24 calibrated with a phase transition temperature of zero to minus ten degrees Celsius at which material 24 transitions from liquid to solid and yields up its heat of fusion, thereby transferring heat to the aqueous solution and helping to prevent freezing.

FIG. 4 is a construction similar to that of FIGS. 1-3 but with containers 20c and 20d of phase change materials installed on the outer surfaces of storage vessel 10. This construction is suited to aftermarket applications in which the exterior of a previously installed storage vessel is readily accessible.

Turning to the structure of FIG. 5, an outer housing 30 and a cap 32 surround an inner storage vessel 34 and form an annular space 36 with the walls of vessel 34. Inner storage vessel 34 contains an exhaust gas treating liquid like aqueous urea solution 17 and annular space 36 is filled with phase change material 24 which is in heat transfer relationship with aqueous urea solution 17. The structure of FIG. 5 operates like the operations of FIGS. 1, 2, and 3 as described above.

As shown in FIG. 6, annular space 36 of FIG. 4 can be divided horizontally by a partition 38 to enable installing one phase change material 40 in the resulting lower compartment 42 and another phase change material 44 having a different phase transition temperature in the upper compartment 46. The phase change materials can complement each other with each helping to control rising temperatures but at differing phase transition temperatures as described above, or one can help control rising temperatures while the other helps control falling temperatures. In an alternative shown in FIG. 7, annular space 36 is divided vertically by a partition 50 into separate compartments for phase change materials that can have different PTTs as described.

Turning to the system of FIG. 8, a diesel engine 60 has an exhaust conduit 62 that delivers engine exhaust to a Selective Catalyst Reduction (SCR) unit 64. Downstream of SCR unit 64 typically are additional exhaust treatment devices 66 and a muffler 68.

An aqueous urea injection system indicated generally by 70 comprises an aqueous urea storage vessel 72 equipped with phase change material and having the structure as described above for any of FIGS. 1-7. Inlet 16 enables adding a treating liquid such as aqueous urea into vessel 72 and outlet pipe 18 enables pumping the liquid via delivery line 73 to a dosing unit 74 located in exhaust conduit 62 and upstream of SCR unit 64. A return line 76 enables returning excess liquid to storage vessel 72.

A urea solution is introduced into storage vessel 72 via inlet 16 and a pump internal to vessel 72 pumps the urea solution to dosing unit 74 which sprays the solution into exhaust conduit 62 upstream of SCR unit 64. Considerable mixing occurs inherently and the urea converts to ammonia that SCR unit 64 uses to materially reduce the amount of NOx in the engine exhaust. A control system and appropriate valves and sensors that are necessary to make the system operate properly and reliably are taught by the art.

FIG. 9 shows an added feature of the system in which return line 76 is encased in a cylindrical container 80 that surrounds line 76 and forms a cylindrical annular space 82 with line 76. Phase change material 84 having a phase transition temperature that typically is calibrated to absorb thermal energy at a temperature of less than one hundred degrees Celsius is installed in annular space 82. As heated aqueous solution returns from dosing unit 74 via return line 76 to vessel 72, phase change material 84 undergoes a phase change and thereby helps to maintain thermal stability. Similarly, delivery line 73 can be encased in a container of phase change material that is calibrated with a phase change material having a phase transition temperature suited to the needs of the system.

A variety of suitable phase change materials are taught by the art. Hydrated inorganic salts suitable for use are disclosed in Hammond U.S. Pat. No. 5,785,884, Lane et al. U.S. Pat. No. 4,585,572, and Lane et al. U.S. Pat. No. 4,613,444, and in the literature along with many paraffins, polyethylene glycols, and additional organic compounds. As guidelines, lithium nitrate trihydrate has a phase transition temperature of 29.9 degrees Celsius and a heat of fusion of three hundred joules per gram., and heptadecane has a phase transition temperature of 21.7 degrees Celsius and a heat of fusion of two hundred thirteen joules per gram. These and many other compounds can be modified chemically to adjust the phase transition temperature by procedures known to the art and modest quantities would handle considerable thermal energy in a typical storage vessel of this invention.

The treating liquid can be made up of any of several formulations of urea, including ammonium carbonate, ammonium bicarbonate, ammonium carbamate, ammonium cyanate, ammonhium salts of inorganic and organic acids, ammelide, or ammeline. Typically a relatively inexpensive commercial urea is used. Aqueous solutions of urea can be used up to solubility limits and selection of the concentration depends on temperature control and other factors, with thirty-five percent urea being typical.

As noted above, packaging the bulky equipment of the required exhaust emissions control system for treating NOx in trucks and other vehicles is a major challenge. This invention offers considerable flexibility in that phase change material can be calibrated to undergo a phase change and absorb thermal energy when the temperature of the liquid increases above a calibrated temperature, or can be calibrated to undergo a phase change and release thermal energy when the temperature of the liquid decreases below a calibrated temperature. Multiple containers with both of these phase change materials also can be used for improved thermal stability. The invention operates passively and does not need additional control equipment. 

1. A storage vessel with improved thermal stability for liquids used to reduce engine exhaust emissions comprising an inlet for admitting a liquid into the storage vessel, an outlet for providing liquid to exhaust emissions equipment of the engine, and phase change material in heat transfer relationship with the liquid, said phase change material being calibrated with a phase transition temperature within an identified temperature range to exchange its heat of fusion with the liquid and thereby stabilize the temperature of the liquid within the identified temperature range.
 2. The storage vessel of claim 1 in which the phase change material is calibrated to undergo a phase change and absorb thermal energy when the temperature of the liquid increases above the phase transition temperature of the phase change material.
 3. The storage vessel of claim 2 in which the liquid is an aqueous solution of urea and the phase change material is calibrated with a phase transition temperature within a range of fifty to one hundred degrees Celsius.
 4. The storage vessel of claim 3 in which the phase change material is high-temperature phase change material, said high-temperature phase change material being calibrated with a phase transition temperature within a range of seventy to one hundred degrees Celsius.
 5. The storage vessel of claim 4 comprising low-temperature phase change material in heat transfer relationship with the liquid, said low-temperature phase change material being calibrated with a phase transition temperature within a range of fifty to seventy degrees Celsius.
 6. The storage vessel of claim 5 comprising additional phase change material in heat transfer relationship with the liquid, said additional phase change material being calibrated with a phase transition temperature within a range of zero to minus ten degrees Celsius to release thermal energy when the temperature of the liquid is within the range of zero to minus ten degrees Celsius.
 7. The storage vessel of claim 1 comprising a sealed container positioned on an interior surface of the storage vessel and in heat transfer relationship with the liquid, said sealed container containing said phase change material, said phase change material being calibrated with a phase transition temperature within a range of fifty to one hundred degrees Celsius to absorb thermal energy.
 8. The storage vessel of claim 1 comprising a first sealed container in heat transfer relationship with the liquid, said sealed container containing said phase change material, said phase change material being calibrated with a phase transition temperature within a range of seventy to one hundred degrees Celsius to absorb thermal energy, and a second sealed container in heat transfer relationship with the liquid, said second sealed container containing phase change material calibrated with a phase transition temperature within a range of fifty to seventy degrees Celsius to absorb thermal energy.
 9. The storage vessel of claim 1 in which the phase change material is calibrated with a phase transition temperature within a range of zero to minus ten degrees Celsius to release thermal energy when the temperature of the liquid is within the range of zero to minus ten degrees Celsius.
 10. A storage vessel with improved thermal stability for liquids used to reduce engine exhaust emissions comprising an inlet for admitting a liquid into the storage vessel, an outlet for providing liquid to exhaust emissions equipment of the engine, an outer housing surrounding the storage vessel and forming an annular space with the storage vessel, and phase change material installed in the annular space and in heat transfer relationship with the liquid.
 11. The storage vessel of claim 10 in which the liquid is an aqueous solution of urea and the phase change material is calibrated with a phase transition temperature within a range of fifty to one hundred degrees Celsius to absorb thermal energy when the temperature of the liquid is within a range of fifty to one hundred degrees Celsius.
 12. The storage vessel of claim 11 comprising a partition that extends through the annular space and divides the annular space into more than one compartment, each of said compartments being suitable for containing phase change material.
 13. The storage vessel of claim 12 in which one compartment contains phase change material calibrated with a phase transition temperature within a range of fifty to one hundred degrees Celsius to undergo a phase change and absorb thermal energy when the temperature of the liquid is within a range of fifty to seventy degrees Celsius, and another compartment contains phase change material calibrated with a phase transition temperature within a range of seventy to one hundred degrees Celsius to absorb thermal energy when the temperature of the liquid is within a range of seventy to one hundred degrees Celsius.
 14. The storage vessel of claim 13 comprising additional phase change material is calibrated with a phase transition temperature within the range of zero to minus ten degrees Celsius to release thermal energy when the temperature of the liquid is within the range of zero to minus ten degrees Celsius.
 15. A method for injecting an exhaust treatment liquid into engine exhaust gases generated by an engine and having an exhaust conduit for conducting the exhaust gases to a catalytic unit comprising providing a storage vessel for the exhaust treatment liquid, introducing exhaust treatment liquid into the storage vessel, stabilizing the exhaust treatment liquid with phase change material being calibrated with a phase transition temperature within an identified temperature range to exchange its heat of fusion with the liquid and thereby stabilize the temperature of the liquid within the identified temperature range, and conducting the exhaust treatment liquid to the engine emissions gases.
 16. The method of claim 15 comprising calibrating the phase change material with a phase transition temperature of seventy to one hundred degrees Celsius to undergo a phase change and absorb thermal energy when the temperature of the exhaust treatment liquid within the storage vessel is seventy to one hundred degrees Celsius.
 17. The method of claim 16 comprising placing second phase change material within the storage vessel and in heat transfer relationship with the exhaust treatment liquid, and calibrating said second phase change material with a phase transition temperature of fifty to seventy degrees Celsius to undergo a phase change and absorb thermal energy when the temperature of the exhaust treatment liquid within the storage vessel is fifty to seventy degrees Celsius.
 18. The method of claim 17 in which dosing equipment for spraying exhaust treatment liquid into the engine exhaust gases is located in the exhaust conduit upstream of the catalyst, and comprising transporting extra exhaust treatment liquid to the dosing equipment, returning excess exhaust treatment liquid to the storage vessel via a return line, and stabilizing the temperature of the returning exhaust treatment liquid within the return line with phase change material.
 19. The method of claim 15 in which dosing equipment for spraying exhaust treatment liquid into the engine exhaust gases is located in the exhaust conduit upstream of the catalyst, and comprising transporting extra exhaust treatment liquid to the dosing equipment, returning excess exhaust treatment liquid to the storage vessel via a return line, and stabilizing the temperature of the returning exhaust treatment liquid within the return line with phase change material.
 20. The method of claim 15 comprising calibrating the phase change material to undergo a phase change and release thermal energy when the temperature of the exhaust treatment liquid is zero to minus ten degrees Celsius. 